1. Field of the Invention
The present invention generally relates to a method of manufacturing integrated circuit devices on semiconductor substrates. More specifically, the present invention relates to a processing method for doping the channel region of a semiconductor device.
2. Description of the Related Art
In many Metal Oxide Semiconductor (MOS) integrated circuit applications, transistor performance is highly dependent on the ability to set and maintain a uniform and stable threshold voltage V.sub.T. One technique for controlling threshold voltage V.sub.T is implantation to properly set the net dopant concentration at the surface of a silicon wafer. In this manner, threshold voltage V.sub.T is selected on the basis of device performance alone. A V.sub.T -adjust implant technique involves depositing and etching a threshold mask, then implanting boron, phosphorus or arsenic ions directly into the substrate or through a thin oxide layer to the regions under the gate oxide of a MOSFET. Boron implantation produces a positive shift in threshold voltage V.sub.T. Phosphorus or arsenic implantation causes a negative shift. The V.sub.T -adjust implant is often performed through an oxide layer with the implant energy selected to place the peak of the implant slightly below the oxide-silicon interface. Following an implant-activating anneal step, the implanted distribution is broadened.
One problem with the V.sub.T -adjust implant is that the dopant profile in a device channel is nonuniform due to the introduction of extra ions into the channel depletion region of the device, causing the width of the channel depletion region to be modified. The non-uniform doping profile changes the long-channel subthreshold characteristics of the device as well as the punchthrough behavior of short-channel devices.
Another problem with the V.sub.T -adjust implant is that the implant depth is difficult to control so that shallow depths are difficult to achieve. A shallow implant is difficult to achieve using the V.sub.T -adjust implant due to limitations on energy of an implant device. For example, boron molecules are very small so that even a minimal implant energy drives the boron molecules an unsuitable depth into the substrate. BF.sub.2 molecules are larger and therefore more easily implanted to a shallower depth although further limitations on implant depth are desired. Increasing implant depth disadvantageously creates channeling effects, creates lattice damage, and increases the voltage drop between the implant and the substrate surface.
What is needed is a technique for doping the channel of a semiconductor device that facilitates control of the impurity concentration within the channel, promotes a shallow dopant depth and improves the uniformity of dopant concentration in the channel.